Optical waveguide filaments are a promising transmission medium recently developed for use in optical communication systems. Such filaments generally consist of an optical fiber having a transparent core having a refractive index n.sub.1 surrounded by a layer of transparent cladding material having a refractive index n.sub.2 which is lower than n.sub.1. Although the theory of optical waveguides has been known for some time, practical optical waveguides that do not absorb an excessive amount of transmitted light have been developed only recently. For example, U.S. Pat. No. 3,659,915 discloses a low loss optical waveguide comprising a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding. The optical signal transmission lines that are employed to transmit optical signals between stations may comprise a single optical waveguide or a bundle thereof.
To establish an optical communication system between a plurality of stations, a variety of interconnection schemes may be utilized. Each station can be "hard wired" to every other station, but when many stations must be interconnected, the excessive amount of optical signal transmission line required causes this method to be undesirable due to both the cost of the transmission line and the space consumed thereby. The stations may be interconnected by a loop data bus which drastically reduces the required amount of optical signal transmission line, but the large number of couplers required in such a system introduces an excessive amount of loss, especially in those systems in which there are many stations.
The optical communication network disclosed in U.S. Pat. No. 3,883,217 to R.E. Love et al. takes advantage of unique properties of optical signal transmission lines and enables the interconnection of a plurality of stations with much less transmission line than that which would be required by hard wiring, and yet it is not plagued by the losses encountered in the aforementioned loop data bus. Briefly, the communication network disclosed in said Love et al. patent, which is incorporated herein by reference, consists of a number of stations, all of which are connected by separate optical signal transmission lines to a common passive coupler which is adapted to receive an optical signal to the optical signal transmission line associated with each of the other stations. The coupler employed in this system should couple light from each optical signal transmission line to the remainder of such transmission lines and should introduce a minimum of loss into the system.
A passive coupler designed for use in the system of the Love et al. patent is taught in U.S. Pat. No. 3,874,781 issued to F.L. Thiel. That coupler comprises an elongated cylindrically shaped rod of transparent material having first and second endfaces that are substantially perpendicular to the axis thereof, the second endface having a light reflecting layer disposed thereon. Support means disposes the ends of the transmission lines in a bundled arrangement in such a manner that the optical waveguides of which the transmission lines are comprised terminate adjacent to the first endface of the rod, the axes of the waveguides being substantially parallel to the longitudinal axis of the rod. Light emanating from any one of the transmission lines diverges as it propagates through the rod. This light reflects from the light reflecting layer and again propagates through the rod to the input end thereof where it is coupled to all of the transmission lines. One of the insertion losses of this coupler is the packing fraction loss which is related to the packing fraction of the optical waveguides at the end of the cylindrically shaped rod, i.e., the ratio of the area of the fiber cores to the area of the entire rod endface. For example, only about 33% of the light uniformly illuminating the end of a bundle of filaments falls on the filament core areas, assuming the filaments are stacked in a close packed array and are circular filaments having an overall diameter of 5 mils and a cladding thickness of 1 mil.